The present invention relates in general to memory-devices, and in particular, to an interleaved memory readable in a synchronous mode or in a random access asynchronous mode with fast access times. The interleaved memory is readable in the synchronous mode by successive locations with a sequential type of access, commonly referred to as a burst mode.
In a standard memory a read cycle is defined from a request of data effected by the input of a new address, to the final output of the bits stored in the addressed location (byte, word, etc.). Internally, the reading process evolves through several steps. These steps start from the acquisition of the new address, to its decoding, to the generation of synchronizing pulses of the sensing circuits, and to the output of the read data.
The ATD (Address Transition Detection) signal recognizes a change of the address input by the external circuitry, and therefore, the new request of access and initiates a new read cycle. After enabling the sense amplifiers by the signal SAenable, an equalization of the sensing circuitry takes place. At the end of which, as timed by the signal EQZ, the effective reading of the memory cells takes place. Finally, after a certain interval of time that may vary from device to device, by way of a signal SAlatch, the recording of the read data into the latches in cascade to the sense amplifiers takes place, from where the read word may be transferred to the output buffers.
In memory devices designed for a synchronous read mode with a sequential type (burst) of access, the reading process exploits the fact that the reading takes place by successive locations. That is, the subsequent memory location to be read, and therefore, its address, is predictable from the address of the location being currently read.
A subgroup of these sequential (burst) synchronous read mode memories is represented by the interleaved memories. A burst access interleaved memory is described in U.S. Pat. No. 5,559,990, for example. In this type of memory, the cell array is divided in two semi-arrays or banks, each having its own read circuitry. The read streams of the two banks are thereafter superimposed according to one of the most commonly followed approaches. They are outphased, i.e., out of phase, from each other. While on one of the two banks or semi-array the steps of evaluation and transfer of the data to the output are being performed, on the other bank or semi-array (the next location to be addressed) a new read cycle may start without waiting for the conclusion of the current read cycle that involves the first semi-array.
In interleaved memories, a basic scheme of which is depicted in FIG. 1, the array is divided into two independent banks or semi-arrays, EVEN and ODD, respectively, each having its own independent read path. Typically, there are two counters (one for each bank) containing the address of the currently pointed memory location. In case of simultaneous reading processes evolving respectively on the two semi-arrays, the least significant bit of the address (A0) supports the multiplexing between the EVEN and the ODD banks. If A0=0, the data coming from the EVEN semi-array will be made available at the output. If A0=1, the data coming from the ODD semi-array will be made available at the output.
As it is commonly known, the reading of the two semi-arrays is carried out according to one of two different approaches. A first approach is simultaneous readings and multiplexing of the outputs. A second approach involves time readings that are out of phase.
According to the first approach, the readings are simultaneous on the two banks. The data read are stored in respective output registers and made available to the outside world in synchronization with an external clock signal. According to the second approach, the readings on the two semi-arrays have an alternate and interleaved evolution over time.
The first approach, though offering a simpler hardware implementation, limits the minimization of the start times of synchronous read cycles. For a better comprehension, it is necessary to consider the basic steps that are performed when passing from an asynchronous read mode to a synchronous read mode.
With reference to the scheme of FIG. 2, and starting the reading from an address X, the latter will be loaded on the EVEN bank counter and on the ODD bank counter, less the least significant bit (A0) of the address. The two counters will point to the same location X of the respective bank or semi-array.
If A0=0: the first read data is relative to the address X of the bank EVEN and the successive read data is the data X of the bank ODD.
If A0=1: the first read data is relative to the address X of the bank ODD and the successively read data is relative to the X+1 address of the bank EVEN.
In the first case, it is sufficient to perform a simultaneous reading of the two banks and multiplex the outputs. In the second instance, it is necessary to increment the counter before starting the reading on the bank EVEN.
Usually, known synchronous memory devices do not make any initial increment and wait for the successive cycle for incrementing both counters, and therefore, read the location X+1 of the banks EVEN and ODD. This makes the times of the first read cycle and of the second sequential read cycle at best equal to the asynchronous read mode time of the memory.
In general, it may be stated that the efficient management of the read processes has a direct influence of the performance of the memory device. Many read-path architectures have been proposed. Known read-path architectures have generally been conceived for responding efficiently to either one or the other of the two modes of operation: asynchronous or synchronous.
If a memory device is designed to be read in an asynchronous mode, it will be generally provided with a rather simple control circuitry for the read data streams. This allows the use of adaptive structures, such as dummy wordlines and dummy sense amplifiers, while leaving the reading circuitry free to evolve as fast as possible in order to achieve the shortest asynchronous access delays.
In contrast, in memory devices designed to function in a burst access mode or in a synchronous read mode, the possibility of making available in output a certain number of words read and stored in advance, permits, after a first asynchronous access, as long as it may be, a series of extremely fast read cycles. In this case though, the control logic must intervene extensively to manage the sense amplifiers which should not be left to evolve freely but be enabled, equalized and read at precise instants established by the control system. Prior European Patent Application Serial No. EP-98830801, filed on Dec. 30, 1998, and Italian Patent Application Serial No. MI99A00248, filed on Nov. 26, 1999, describe burst-mode EPROM devices with the above characteristics. These patent applications are both incorporated herein by reference in their entirety, and are assigned to the assignee of the present invention.
The access mode in a reading phase of operation is set to a specific protocol of use of two of the external protocol signals. These two signals are the address input latches enabling signal ALE and of the read stimulation signal RD.
The counters of the two semi-arrays, or the counter of the first bank and a more simpler register which functions as an address counter of the second bank, are incremented distinctly from one another. This is different from what is commonly done in interleaved memory devices. The readings are thus out of phase on the two banks from the first (asynchronous) read cycle. In this way, the memory device of the invention is perfectly able to switch to a synchronous mode reading phase at any time, which practically cuts in half the access time to such a mode.
The two different reading processes, according to an asynchronous random access mode and according to a synchronous burst access mode remain congruent with each other, having an alternate and interleaved evolution in time, as described in European Patent Application No. 00830068.3, filed on Jan. 31, 2000. This application is incorporated herein by reference in its entirety, and is assigned to the assignee of the present invention.
The architecture disclosed in the referenced patent application provides for optimal performances, uncompromised in any measure by the bivalent character of the device, whether commanded to function as a standard asynchronous random access memory or as a synchronous burst access memory. The control circuit of the memory recognizes the type of access and reading mode that is required by way of a specific protocol of use of two external commands, namely the address latch enabling signal ALE and the read simulation signal RD signal.
The start of a standard read cycle (asynchronous mode) takes place as customary when the ALE signal assumes a high logic level 1. Upon the switching to a logic 1 of the ALE signal, the memory acquires (in its input latches) the desired address and simultaneously starts up the sense circuitry of both banks.
Of course, a first cycle will always be asynchronous and the independent arrays of sense amplifiers of the two banks are simultaneously activated. If the ALE signal does not return to its rest condition, that is to a logic 0 state, the two banks of sense amplifiers will complete their respective readings, each evolving according to its own self-adapting circuits. This is the same manner as in a standard asynchronous memory.
At the end of these readings, only the bank currently in a condition of priority, determined by the value of the least significant bit of the address, will be enabled to place the read data on an internal data bus. When ADD less than 0 greater than =0, this establishes the priority of the EVEN bank, and when ADD less than 0 greater than =1, this establishes the priority of the ODD bank.
Under these conditions, the reading stimulation signal RD behaves as an active low Output Enable command, such that when at a logic 0 level, the data of the bank currently in priority is transferred to the output buffers, and therefore, made available to the external world.
If the ALE signal switches back to a logic 0 level during the very first read cycle or thereafter during the nth asynchronous random access read cycle, the control circuit interprets such an occurrence as a request for a change to a burst mode of access and to a synchronous read mode of the memory. In such a case, the control circuit generates a first increment pulse for the address counter (or optionally for the functionally equivalent register) of the bank which currently is not in priority.
In so doing, the reading of the data on the bank currently in priority is left to evolve, considering that this bank will be the first to be called to provide its data to the output of the memory, Simultaneously, the incremented address for the successive reading to be done on the other bank is prearranged (start of the synchronous burst interleaved reading mode).
The same increment pulse for the address counter (or register) of the bank currently not in priority stimulates also the respective reading circuitry of the bank. A specially modified ATD generator generates upon stimulation by the address counter incrementing pulse a dummy ATD pulse that is due to internal stimulation, and which is exclusively conveyed to the bank not in priority, and to its decoding and sensing circuits.
In this way, the array of sense amplifiers of the bank not in priority, the relative sense control circuit and equalization dummy structures, etc., will interpret the address counter incrementing pulse as a normal asynchronous read cycle request and will restart. This is while the sense amplifiers of the bank currently in priority, being completely independent from those of the other bank, will continue to evolve in their reading process.
The control circuit of the memory will continue to monitor the signals coming from the sense control of the bank in priority. As soon as the sense control circuit provides the SAlatch signal for transferring the read data to the latches immediately in cascade of the respective sense amplifiers, the control circuit will generate a pulse commanding the loading of data on the output, i.e., a LOAD pulse. In this way, the functioning enters in a burst interleaved access mode.
The end of the LOAD pulse establishes the availability of the bank that has just terminated a read cycle to start a new read cycle. Therefore, a new address counter incrementing signal will be generated only for the counter (or equivalent register) of such a bank, which besides stimulating the generation of a sequentially updated internal address, will cause the generation of a new ATD pulse by internal stimulation (dummy). This is exclusively conveyed to the circuitry of such a bank.
These succession of address counter incrementing pulses have also the function of commuting the priority between the two banks in an alternate manner. In fact, because the first bank has terminated a read cycle and is about to start a new one, the bank that was not in priority and that in the meanwhile had restarted on its up-dated address, assumes priority.
Therefore, the control circuit of the memory will again wait for information coming from the asynchronous and self-adapting structures of sense control of such a bank as well as for an authorization by the external command RD to output the data of such a bank by the generation of a new LOAD pulse. From hereon, the steps described above may repeat indefinitely, alternating the sequence.
According to a preferred embodiment, the circuit that detects transitions in the latches acquiring the externally requested addresses generates a detection signal ATD even upon a switching of the outputs. This is during a phase of reentering from a state of standby with the external command ALE=0, and besides the acquisition of a new address from outside during a phase of random access or upon the incrementing of one the other address counters of the two banks during a phase of sequential access. This provides for a useful resumeandrecovery functionality, such as for producing in output the last read data before entering the phase of standby, or the data relative to the sequentially successive address. This depends on the logic state of the RD command at the moment of interrupting the sequential reading and entering in a phase of standby of the memory.
From the above mentioned reasons it is clear the importance to ensure an address updating that is as rapid as possible when an increment pulse is detected during a synchronous read access (burst mode) for selecting a new memory location. Commonly, the address counters are binary counters in which the carry propagation chain determines or limits its speed.
According to known techniques, the commonly used binary counters are based on a Carry Look Ahead architecture (CLA), according to a circuit diagram as the one depicted in FIG. 5. Such a common architecture of binary counter in the form of a CLA adder, besides requiring the integration of a relatively complex hardware, has speed limits imposed by the delays of the carry propagation chain.
In view of the foregoing background, the present invention provides an excellent approach to the problem of the carry propagation in binary counters by reducing in a non-negligible manner the propagation time by using a particular carry calculation network.
Such a network is formed by an ordered set of carry generators, with each one calculating a carry for a respective group of stages of the counter. These generators are substantially carry calculation networks having a lesser number of stages than the counter.
While the commonly used address counters are provided with a carry calculation network ideally obtained by connecting in cascade several carry generators, the counter of the invention has a carry calculation network obtained by connecting in parallel such generators by AND gates.
The first carry generator of the counter of the invention is coupled to the same addresses of the first carry generator of the known counters. On the contrary, the successive generators of the invention do not receive, at the respective input of the carry bit, the bit CARRY related to the most significant bit of the preceding generator in the ordered group as in the known structures. Instead, they receive a carry bit quickly calculated by carrying out the logic AND of all the addresses of the previous generator.
The carry bit is quickly calculated and coincides with the CARRY calculated by the preceding generator, because a CARRY is produced only when all the respective input bits are 1. The main advantage of this architecture includes the fact that the generators work in parallel, with a definite saving of time. In contrast, the carry generators in known counters work sequentially, that is the nth generator can work only when the nxe2x88x921th generator that precedes it in the ordered group has terminated its own cycle.